Diathermy is the use of special electrical currents to raise the temperature of living tissue for therapeutic purposes. It is sometimes known as hyperthermy, particularly in the medical sector.
Equipment for diathermy by conduction current is very effective in many applications in the field of electromedicine and electroaesthetics. Electroscalpels are also sometimes referred to as diathermy appliances.
It is advantageous for diathermy equipment and electroscalpels to work with high-efficiency amplifiers (from the point of view of energy consumption), but in these circumstances they usually emit multiple electromagnetic signals due fundamentally to parasitic elements which interfere with other electronic pieces of equipment.
These pieces of equipment for diathermy by conduction and electroscalpels usually function with sinusoidal signals like the one in FIG. 1. FIG. 2 shows the frequency spectrum of the signal of FIG. 1 using the Fourier transform which is another way of representing the signal in the frequency domain, the X axis showing the frequency and the Y axis showing the amplitude of the signal.
The increase of temperature of the living tissue by diathermy is obtained by transmitting energy to the tissue by two methods: by induced currents (electrodes not in contact with the tissue) or by conduction currents (electrodes in contact with the tissue).
In general, the signal frequency applied in the contactless connection method must be very much higher than the signal frequency applied in the contact connection method.
In diathermy by conduction two electrodes are contacted with the living tissue in such a way that a current flow occurs between the two electrodes and passes through the tissue that it encounters in its passage. The current flowing through the tissue causes the temperature to rise by the Joule effect, due to the electrical resistance of said tissue.
In equipment for diathermy by conduction the electrodes are connected by contact. There are two application methods: one is known as the capacitive method and the other as the resistive method.
The electrodes used in diathermy by conduction are normally asymmetric. In this case, and due to the current density, the greatest rise in temperature occurs in the tissue nearest the active electrode (the smaller one). In the capacitive method, the two electrodes are metal, but one of them has an insulating layer. In the resistive method, the two electrodes are metal with no insulation.
In electrosurgical equipment, such as electroscalpels, the current density is so high at the point of contact between the active electrode and the tissue that cutting, coagulation or fulguration of the tissue occurs.
It is very difficult to comply with EMC standards mentioned above when using this type of equipment. In fact, standard IEC 60601-2-2, which must be met by electroscalpels, states that EMC tests must be carried out with the equipment connected, but with the output power at zero, since, at the time, it was accepted that it was very difficult to comply with EMC standards when the electroscalpel is used for cutting, fulguration or coagulation, and since the equipment is used during a surgical operation for a relatively short time and the benefit to the patient is very high.
Equipment for diathermy by conduction operate in a similar way to an electroscalpel, but with a much larger active electrode, so that the current density J in the contact area with the tissue is much lower, whereas the current I which flows through the tissue (of up to 3 A effective R.M.S. value), the output voltage V (of up to 800 V of effective R.M.S. value) and the signal frequency (between 0.4 MHz and 3 MHz) are of approximately the same order of magnitude. From the EMC point of view, these voltages, currents and frequencies are relatively high, and make it very difficult to comply with the EMC standards in different countries. As there are no specific standards for this type of equipment, as there are for electroscalpels, equipment for diathermy by conduction must meet the general EMC standard for medical equipment, EN 60601-1-2 in Europe. This states that EMC must be measured with the equipment in the worst possible condition, which is, at the maximum output power, unlike electroscalpels which must be measured with the power at zero, which is a very much easier situation in which to meet the EMC requirements.
Any pure and periodic non-sinusoidal signal may be broken down into multiple sinusoidal signals known as harmonics (of differing frequency, amplitude and phase), which can be calculated from the Fourier transform of the periodic signal.
A method for minimising electromagnetic interference consists of the signal on which the amplifier works being pure sinusoidal, for example a class A amplifier, but in this case the maximum theoretical efficiency is 50% with the consequent energy loss and heating of the equipment.
Some high-efficiency amplifiers such as those in class C, D, E or F, for example, may lead to theoretical efficiencies of up to 100%. These amplifiers are based on generating a signal that ideally is squared (but may be trapezoidal or quasi-trapezoidal) or a pulse signal, and filtering the fundamental signal with the intention of achieving maximum attenuation of undesired harmonics, normally with a second order LC filter, which may be serial or parallel, but no filter has been described in the prior art like the one described in this patent for this type of amplifier.
In practice, the efficiency of these amplifiers is less than the theoretical efficiency due to losses in the components and/or to non-matching of impedances.
FIG. 3 shows a squared signal. FIG. 4 shows that the frequency spectrum of said squared signal contains many other higher frequency signals (harmonics). These signals are undesired because they may connect to the network cable or because they may emit in the form of radiation through the patient's cables, which may more easily cause electromagnetic interference in other equipment.